1. Field of the Invention
This invention provides the means and mechanism by which to produce a steady source of high-energy neutrons which, in addition to the multiplication and efficient transformation of the radioactive energy of the primary driver isotope, can also be changed in strength through simple adjustments to the physical layout of the multiplier assembly. The resulting neutron source has many practical uses including, but not limited to: startup source for a nuclear reactor, non-destructive testing of materials, neutron activation analysis, sample moisture analysis, oil well logging, medical treatment of cancer, explosive detection, metal fatigue detection, and other real-time evaluations of chemical composition and moisture content of process streams such as combustion optimization in power plants and cement kilns.
2. Description of Related Art
Multiple neutron sources (emitters) are generally required in order to safely start up a nuclear reactor core. The reactor startup sources used for this purpose are referred to as “primary sources” and “secondary sources.” Primary sources are self-contained sources of neutrons that provide neutrons without the need for external power or irradiation from the reactor itself. Secondary reactor startup sources are universally made of initially non-radioactive driver materials uniformly mixed with beryllium. The secondary source driver material (typically antimony) is non-radioactive for manufacture. As a result, the secondary source does not produce a neutron source until the driver material is irradiated in a nuclear reactor. The secondary source produces neutrons as a result of the interaction of high energy gamma radiation from the radioactive decay of the driver material with the beryllium. Typical of the current art primary source driver materials, all used in combination with beryllium, are strong alpha particle emitting isotopes of polonium, radium, plutonium, americium or curium. The only material that is a practical primary source for commercial applications without the use of admixed beryllium is californium-252 or 252Cf.
Descriptions of producing “secondary source” radio-isotopes within nuclear reactors is generally described by Ransohoff et al. and Bodnarescu (U.S. Pat. Nos. 3,269,915 and 3,396,077, respectively). A description of use of “primary sources” and the general use of neutron sources is described, in detail, by Impink, Jr. (U.S. Pat. No. 4,208,247—issued in June 1980, hereinafter “Impink”), where, preferably, plutonium-238 and beryllium are encapsulated in an alloy that does not allow transmission of thermal neutrons, that is, essentially “black” to thermal neutrons, such as pure cadmium; 65% silver/cadmium or 80% silver/15% indium/cadmium.
A reactor start-up neutron source is used to safely assist the initiation of nuclear chain reaction in the initial core loading of nuclear reactors. A reactor startup source is required for safe startup of an initial core containing only fresh unirradiated nuclear fuel because the neutron population density from all sources (e.g., spontaneous fission of the fuel, cosmic radiation, deuterium photoneutrons) is insufficient for reliable monitoring of the reactor neutron population to assure safe reactor start-up. Low neutron fluxes occur in nuclear reactors with initial cores with only mildly radioactive fuel or after prolonged shutdown periods in which the irradiated fuel has decayed thereby reducing the inherent neutron source of the reactor from the previously mentioned mechanisms. Fixed reactor primary and secondary startup neutron sources provide a population of neutrons in the reactor core that is sufficient for the plant instrumentation to reliably measure and therefore provide reactor power and reactivity information to the reactor operator to enable a safe reactor startup and also to the reactor protection system to override the operator and halt the reactor startup if an unsafe situation is detected. Without reactor startup neutron sources, the reactor could suffer a fast power excursion during start-up before the reactor protection system could intervene to terminate the startup. The start-up sources are typically inserted in regularly spaced positions inside the reactor core either in place of some of the fuel rods or within structures inside the reactor core.
In addition to the startup of nuclear reactors, neutron sources have many uses in other industrial applications. These industrial uses for neutron sources typically involve the use of the neutron source to create radioisotopes in the vicinity of the source after which the unique nuclear decay characteristics of the radioisotope(s) so created in the process being evaluated are measured and concentrations or compositions are inferred from the measurements in a process typically referred to in the art as neutron activation analysis. The resulting industrial applications include but are not limited to: non-destructive testing of materials, neutron activation analysis, sample moisture analysis, oil well logging, medical treatment of cancer, explosive detection, metal fatigue detection, and other real-time evaluations of chemical composition or moisture content in process streams such as combustion optimization in power plants and cement kilns.
Impink (cited previously) further teaches that (at the time of the patent), neutron sources for commercial reactors have been positioned within the nuclear core, and remained within the core, during at least one entire operating cycle. The sources maintained a fixed position. In reactors, sources are inserted in selected fuel assemblies and extend within fuel assembly guide thimbles designed to provide structure for the fuel assembly and provide guidance for the insertion of control elements into the reactor. The sources are also disposed in assemblies close to the core periphery so as to be positioned within the detection range of the detection and monitoring apparatus outside of the reactor vessel.
Beryllium is a light weight, strong but brittle, light grey alkaline earth metal. It is primarily used in non-nuclear applications as a hardening agent in alloys, notably beryllium copper. Structurally, beryllium's very low density (1.85 times that of water), high melting point (1287° C.), high temperature stability and low coefficient of thermal expansion, make it in many ways an ideal high-temperature material for aerospace and nuclear applications. Commercial use of beryllium metal presents technical challenges due to the toxicity (especially by inhalation) of beryllium-containing dusts. Beryllium produces a direct corrosive effect to tissue, and can cause a chronic life-threatening allergic disease called berylliosis in susceptible persons.
In the nuclear area, beryllium is an extremely unusual element in that essentially all naturally occurring beryllium is of the 9Be isotope which has a very low binding energy (1.69 MeV) for its last neutron. The result of this peculiar aspect of the nuclear physics of beryllium is that, when excited by radiation more energetic than the threshold energy shown below, the 9Be disintegrates as shown below by neutron emission and forms the much more stable helium or carbon atoms.9Be4+4He2→12C6+1n0Eα=0 (exothermic)9Be4+γ→2·4He2+1n0Ey≧1.6 MeV9Be4+1n0→2·4He2+2·1n0En≧1.6 MeV
Californium (element 98) is a rare and exclusively man-made element that is synthesized by long term irradiation of other rare man-made isotopes such as plutonium or curium in specialized high flux reactors specifically designed to produce high-order actinide isotopes. Californium (Cf) is used exclusively for applications that take advantage of its strong neutron-emitting properties. The 252Cf isotope is, by far, the most widely used isotope of californium for neutron sources due to its high source strength, production yield and relatively long half life. There are currently only two facilities in the world that currently synthesize and separate 252Cf. At this time, ˜90% of the world's annual production of ˜200 milligrams is produced at the fifty year old High Flux Isotope Reactor at the Oak Ridge National Laboratory in Tennessee. The 252Cf produced in the reactor is initially purified at the reactor site by separating the 252Cf from all of the other actinides and fission products that result from the target irradiation in a complex radiochemical process that is performed remotely in a hot cell laboratory. The separation process is concluded by coating an inert material wire, foil or other form with the 252Cf chemical compound from the separation process and placing the resulting form in a cask that shields the resulting 252Cf source material, thereby allowing the material to be removed from the hot cell laboratory. The high neutron strength of 252Cf makes it necessary for any source manufacturing subsequent to the separation of the Cf from all of the other actinides and fission products to be done remotely in a well shielded facility to protect the manufacturing staff. As a result, it is only practical to employ simple manufacturing processes in the manufacture of neutron sources using 252Cf. Even in view of previous patents cited, there seems no logical reason to try to add anything to californium as a neutron source as it is already the strongest source of neutrons by weight of any available radioisotope.
Referring now to prior art FIG. 1, there is shown one embodiment of a typical thermal nuclear reactor including a sealed reactor vessel 10 housing a nuclear core 12 comprised of a plurality of fuel assemblies 14 (shown in FIG. 2A). A reactor coolant, such as one including water, enters the vessel through inlet nozzles 16, passes downward in an annular region between the vessel and a core support structure, turns and flows upward through a perforated plate 20 and through the core 12 and is discharged through outlet nozzles 22.
A fuel assembly 14 is shown in prior art FIG. 2A and includes a plurality of fuel pins 24, containing nuclear fuel pellets 26, arranged in a bundle. The assembly also includes a plurality of guide thimbles 28 which provide skeletal support for the assembly and which are sized to removably receive control rods 29 of control elements 30, positionable above and within the core area by means such as electromagnets 32 which act upon shafts 34 (FIG. 1) removably connected to the control elements 30.
The neutron flux within the core is continuously monitored by detection apparatus such as the neutron detectors 36 (FIG. 1) which are located at an elevation aligned with the elevation of the core 12. The detectors, located external to the vessel, may be fixed or laterally movable by positioning bars 38.
The guide thimbles 28 of the fuel assemblies 14, in addition to receiving control rods 29, shown in FIG. 2A, are sized to receive neutron sources capsules shown in FIG. 2B. The capsules contain a neutron emitting source 44.
The source 44 includes a major mass of fast neutron emitting material, encapsulated and held in place by cladding 48. The preferred source material, for current art reactor startup sources is 252Cf due to a combination of factors including source strength. Nonetheless, 252Cf source material is extremely expensive and only available in limited quantities, so minimizing the requirements of these materials is very important. The optimal solution for a primary source is one that minimizes the amount of 252Cf required to accomplish the required function. Additionally, the lifetime of a neutron source is determined by the minimum source strength that achieves the required function. Therefore, it is one of the main objects of this invention to make more efficient use of the 252Cf to either reduce the amount of 252Cf required for a source or to extend the useful lifetime of a given amount of 252Cf.